The National Academy of Sciences asked a few engineers how to reduce gas consumption in cars with available technology. Basically it costs money and hybrids still aren't viable.

This is a really good article about (theoretical) congestion pricing in NYC and how it would work. A Harvard math grad put together a spreadsheet detailing the negative externalities imposed by car drivers on the city; which is exactly what congestion pricing aims to do - give incentives to people to make choices that impose less costs on others.